1. Field of the Invention
This invention relates to fabrication of MOS circuits, and specifically to fabrication of MOS circuits by the isoplanar process as described in U.S. Pat. No. 3,648,125, entitled "Method of Fabricating Integrated Circuits with Oxidized Isolation and the Resulting Structure" issued Mar. 7, 1972, to Fairchild Camera and Instrument Corporation.
2. Description of the Prior Art
Conventional techniques for the fabrication of silicon gate integrated circuit structures typically involve the use of boron to form P+ regions in a silicon substrate. In one conventional technique, a polysilicon island is formed on the surface of a region of thin silicon dioxide formed on monocrystalline silicon. The oxide overlying selected regions of the monocrystalline silicon wherein source and drain regions are desired, typically in the immediate vicinity of the polysilicon island, is removed and boron oxide is deposited on the exposed monocrystalline silicon. During this processing P+ regions are formed where boron from the oxide diffuses into the underlying silicon. Conventionally, as the next step of the procedure, the boron oxide is removed and an insulating layer is deposited. Openings are then formed in the insulating layer for electrical connections to the P+ source and drain regions in the monocrystalline silicon.
Unfortunately this procedure by which prior art MOS circuits have been formed is beset with a number of problems. For example, structures formed by the above described technique are susceptible to photoengraving defects which lead to unwanted electrical connections between the metal interconnections and the polysilicon islands and the source or drain regions. Further, unwanted polysilicon gate-to-source or gate-to-drain electrical connections occur frequently.
It is now known that removal of the boron oxide which is used to form the source and drain regions, as in the conventional silicon gate process described above, can significantly increase the density of unwanted gate-to-source or gate-to-drain electrical connections. Although the reasons for the above described deficiencies are not fully understood, the resulting undesirable effects are nevertheless well known.